Appendix V. 977 machine tools for cutting metals, because when heated by the frictional resistance of the cut, they approximately return to their original condition of hardness. But makers generally forbid the heating of the tool at any time beyond a bright cherry red, which is about 1500° to 1550° F. Now the higher we can allow the steel to be heated during forging, so much higher proportion- ally can it be allowed to overheat when cutting without drawing the temper, a fact which has led to the introduction by the Bethlehem Co. of cutting steel made on the Taylor-White process, where the higher permissible overheat gives increased value to the speed or the cut as may be desired. In this process the composition of the steel may be as follows : Chromium Tungsten I to 6 % Molybdenum i to 6 % Carbon 85 tO 2% but the best results are obtainable when there is i % Chromium with 4 % Tungsten ; or? as an alternative, i% Chromium with 2% Molybdenum; or, as a further choice, i% Chromium, with 2% Tungsten, and with i% Molybdenum, the proportion of carbon within the limits given being immaterial. In addition to careful composition, the steel must receive a special treatment, which consists of raising the temperature (say during forging) to between 1500° and 1700° F., the surface being protected with powdered slag, then (after forging) letting down rapidly, though steadily, by placing in a bath of lead till a temperature of 1240° F. is reached It is important that not the least rise of temperature be allowed at this stage, however brief. The heat should now be suffered to remain at between 700° and 1240° for the space of about five minutes, which may be done while cutting if desired; and with steel thus made and treated, a cutting speed of 150 feet per min. is easily practicable. The outer surface is generally damaged in heating, and should be ground off.